Three elements are involved in seeing or perceiving color: a light source, an object, and an observer. A framework of describing human color perception according to these three elements is sometimes referred to as the visual observing situation. To build an instrument that quantifies human color perception, each item in the visual observing situation may be characterized.
The first element of the visual observing situation is a light source. A light source is a physical source of light. The visible portion of the electromagnetic spectrum is defined by the International Commission on Illumination (CIE) as 360 to 780 nm. A plot of the relative energy at each wavelength creates a spectral power distribution curve that quantifies the characteristics of the light source. A CIE illuminant is a standard table of numbers representing relative energy versus wavelength for the spectral characteristics of light sources. Some common illuminants and their CIE abbreviations are as follows: Incandescent (A), Average Daylight (C), Noon Daylight (D65), and Cool White Fluorescent (F2). By representing a light source as an illuminant, the spectral characteristics of the first element of the visual observing situation is quantified and standardized.
The second element of the visual observing situation is an object. Objects modify light. Colorants such as pigments or dyes that are in or on the object selectively absorb some wavelengths of incident light while reflecting or transmitting other wavelengths. The amount of light that is reflected, absorbed, or transmitted by the object at each wavelength can be quantified. This can be represented as a spectral curve. By measuring the relative reflectance or transmission characteristics of an object, the second element of the visual observing situation becomes quantified. Relative reflectance, or reflectance factor, is defined as the relative amount of energy measured on an arbitrary sample at a fixed geometry, in reflection, with respect to a known white sample similarly used to define the top-of-scale of the measurement. This is important to distinguish as reflectivity is also a function of angle and total reflectance would require a hemisphere to collect all angles. A device which measures relative reflectance or transmittance as a function of wavelength is typically a spectrophotometer.
The third element of the visual observing situation is the observer, which is often but not necessarily a human. A human eye has structures referred to as rods and cones. Cones are responsible for color vision and have three types of sensitivity: red, green, and blue. The CIE experimentally measured the ability of the human eye to perceive color. The experimentally derived x-bar, y-bar, and z-bar functions became the CIE 1931 2° Standard Observer. The functions x-bar, y-bar, and z-bar quantify the red, green, and blue cone sensitivity of an average human observer. An updated standard was later produced and is referred to as the 1964 10° Standard Observer. This is the standard recommended for use today by the CIE.
In science and industry, the trifecta of light source, object, and observer becomes the trifecta of light source, sample, and spectrophotometer. The CIE X, Y, and Z tristimulus color values are obtained by multiplying the illuminant, the reflectance or transmittance of the object, and the standard observer functions. The product is then summed for all wavelengths in the visible spectrum to give the resulting X, Y, Z tristimulus values.
A colorimetric spectrophotometer may comprise a light source, a diffraction grating, a diode array, and a processor. The instrument may be configured to produce CIE X, Y, Z color values for a sample. Briefly, the light source illuminates the sample being measured. Light reflected by the objects is passed to a diffraction grating which breaks it into its spectral components. Much of the diffracted light falls onto the diode array which senses the amount of light at each wavelength. The spectral data is sent to the processor where it is multiplied with a user-selected illuminant and observer tables to obtain CIE X, Y, Z color values.
The CIE X, Y, Z value system is a color scale. When describing color, the CIE X, Y, Z values are not easily understood (they are not intuitive). Other color scales have been developed to better relate to how humans perceive color, simplify understanding of the metrics, improve communication of color, and better represent uniform color differences. All colors can be organized in three dimensions: lightness, chroma or saturation, and hue. Hunter L,a,b color space is a 3-dimensional rectangular color based on Opponent-Colors Theory with the following dimensions:
L (lightness) axis: 0 is black, 100 is white, and 50 is middle gray
a (red-green) axis: positive values are red, negative values are green, and 0 is neutral
b (blue-yellow) axis: positive values are yellow, negative values are blue, and 0 is neutral
The opponent-colors have been explained physiologically by the organization of cone cells into what are called receptive fields in the fovea of the human eye. A receptive field provides a number of inputs from the cone cells (both positive and negative) that can interface with ganglion cells to produce spatial edge-detection for red-green and blue-yellow stimuli. The spectral distribution for these receptive fields correlates well with a, b.
There are two popular L,a,b color scales in use today: Hunter L,a,b and CIE L*,a*,b*. While similar in organization, a color will have different numerical values in these two color spaces. Both Hunter L,a,b and CIE L*,a*,b* scales are mathematically derived from CIE X,Y,Z values. Scales of chroma and hue are also functions of a* and b*; where chroma is the scalar magnitude, ((a*)2+(b*)2)1/2, and hue angle is represented by the arc tangent of (b*/a*).
Color measurement is employed in industry and in education according to color differences. Color differences are calculated as sample-standard values. According to the CIE L*,a*,b* color scale,
If delta L* is positive, the sample is lighter than the standard. If delta L* is negative, the sample is darker than the standard.
If delta a* is positive, the sample is more red (or less green) than the standard. If delta a* is negative, the sample is more green (or less red) than the standard.
If delta b* is positive, the sample is more yellow (or less blue) than the standard. If delta b* is negative, the sample is more blue (or less yellow) than the standard.
Total color difference (delta E*, or ΔE*) is based on the L*,a*,b* color differences and was designed to be single number metric for PASS/FAIL decisions in industry. Delta E* is determined as the square root of the sum of the squares of L*, a*, and b*:ΔE*=√{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}
Thus far color of an object or sample has been generally ascribed to pigments or dyes. Other qualities of an object also play a role in color, further complicating its measurement and characterization. Surface characteristics and geometry play an important role in color.
One surface characteristic of samples is reflectance. For opaque materials, most of the incident light is reflected. For translucent materials, most of the incident light is transmitted. Reflectance make take either of two forms. Diffuse reflection involves non-directional reflected light. This light is scattered in many directions. Specular reflection is reflection of light by which the angle of reflection matches the angle of incidence of the incident light striking the surface of the object.
Color is seen in the diffuse reflection, and gloss is seen in the specular reflection. The reflection at the specular angle is generally the greatest amount of light reflected at any single angle. From air-to-glass at low angles of incidence, specular reflection represents less than 4% of total incident light. For a 60° angle (as in gloss geometry), the reflection of polished glass is ˜10%. The remaining light is transmitted or absorbed with almost no diffuse reflection.
Richard Sewall Hunter, a pioneer in color and appearance, identified six visual criteria for defining a gloss scale. These are specular gloss, contrast gloss, distinctness-of-image (DOI) gloss, absence-of-bloom gloss, sheen, and surface-uniformity gloss. In the color industry, “instrumental gloss” is the most common form of gloss measurement and correlates with Hunter's specular gloss criteria. The ratio of diffuse reflection (45° to the angle of incidence) to specular reflection (equal to the angle of incidence) if subtracted from unity is a measure of contrast gloss. An exemplary geometry for measuring instrumental gloss on most samples is 60° (i.e., 60/60, defined with respect to the surface normal of the sample). Another geometry (30/30) has combined multiple field angles with a diode array to quantify reflection haze and distinctness of reflected image (DORI). Correlates of a perceived gloss scale in a color appearance model (CAM) may be referred to as visual gloss.
Surface texture of samples can greatly affect perceived color. Samples which have exactly the same color to a spectrophotometer, but which have different surface textures, will appear to have different colors to a human observer. Surfaces may generally be described as glossy or matte. Glossy surfaces appear darker or more saturated. Matte surfaces appear lighter and less saturated. Increased surface roughness affects perceived color such that it appears lighter and less saturated. This is caused by mixing diffuse reflectance (where humans see pigment color) with increased scatter from specular reflectance (white). The rougher the surface, the greater the scatter of the specular reflectance.
Other surface characteristics such as complex spatial patterns also affect perceived color. The S-CIELAB model was designed as a spatial pre-processor to the standard color difference equations, to account for these complex color structures. This model was a first step for simulating color appearance. Since the opponent-color spaces like L*, a*, b* are differentiable, there exists a direct correlation to the spatial receptive field and spatial gradients of the L*, a*, b* values.
Instrument geometry defines the arrangement of light source, sample plane, and detector. There are two general categories of instrument geometries: Bi-directional (45°/0° or 0°/45°) and diffuse (d/8° sphere). Bi-Directional 45°/0° geometry has illumination at a 45° angle and measurement at 0°. The reciprocal, 0°/45° geometry, has illumination at 0° and measurement at 45°. Both directional geometries by definition exclude the specular reflection in the measurement. This is sometimes indicated in numerical tables by the phrase, “specular excluded”. Bi-Directional geometry measurements provide measurements that may correspond to visual changes in sample appearance due to changes in either pigment color or surface texture. To reduce the directionality of an arbitrary sample, the 45° illumination or detection may be revolved circumferentially around the sample in at least 12 equally spaced locations.
Diffuse (sphere) geometry instruments use a white coated sphere to diffusely illuminate a sample with 8° (d/8°) viewing. Measurements on a diffuse sphere instrument can be taken with the specular included or specular excluded. Specular included measurements negate surface differences and provide values which correspond to changes in actual color (as opposed to perceived color). Specular excluded measurements negate specular reflectance on very smooth surfaces, measuring only diffuse reflectance. For illustration, as between two surfaces painted with the same red paint, one surface having a matte finish and the other surface having a high gloss finish, the specular included measurement indicates no color difference. It quantifies only colorant differences and negates differences in surface finishes. In the specular excluded mode, the readings quantify appearance differences, similar to those from the direction (0°/45°) geometry instrument. Most diffuse geometry measurements are taken in the specular included mode.
Color appearance models (currently) describe qualities such as lightness, brightness, colorfulness, chroma, saturation, and hue. They may also be extended to include a gloss scale.
All CAMs rely on an opponent color space such as L*, a*, b*. The L*, a*, b* space, in particular, already quantifies lightness, chroma, saturation, and hue. For this reason, it is well-suited for application in CAM. The lightness and chroma can be scaled to provide brightness and colorfulness.
Glossmeters, spectrophotometers, and other devices employed in optics are traditionally independent instruments. These devices may be specially tailored to detect and characterize very specific qualities of the visual observing situation (e.g., gloss, reflectance, measured color, perceived color, and texture).
While specialized instruments exist for characterizing and quantifying color, achieving high levels of accuracy and reproducibility is difficult for when the instruments are subjected to use with a variety of different sample types or with heterogeneous samples. One sample type to the next (e.g., a cracker versus a cookie) may dramatically different in characteristics which can affect color measurement. Within even a single sample, properties may differ due to sample heterogeneity (e.g., variable shape, color, and size of parts of a chocolate chip cookie, or multiple cookies separated by aluminum space on a tray). Instruments and methods are needed which offer adaptive parameters and operation to accommodate sample type differences and sample differences without compromising accuracy and reproducibility of the color measurements.